The present invention relates to a method and circuit for use in trimming the frequency of an oscillator which is particularly adapted for use in VLSI technology.
In fabricating oscillators on VLSI chips it is standard to fabricate a resistor and capacitor which have, in combination, an RC time constant that determines the frequency of the oscillator. Because of differences present in the fabrication process of the chips it is usually necessary to provide for some means of trimming the oscillation frequency of the oscillator. A standard approach for trimming the frequency of an oscillator is to somehow change the value of the capacitor or the resistor through which the capacitor is charged. Such a technique, however, is quite imprecise because of the parasitic impedance of the switching devices.
Accordingly, it is an object of the present invention to provide an improved circuit for trimming the frequency of oscillation of an oscillator. It is a further object to provide a circuit in which the frequency of oscillation of an oscillator can be changed more accurately than hitherto known circuits.